FUNCTION OF CHI ASM AT A 495 



after pachytene of chromosomes that have been paired at pachytene 

 wherever crossing-over and chiasma-formation have failed. This 

 happens in short chromosomes produced by fragmentation (Ch. V), 

 in the odd chromosomes of triploids, and through partial failure of 

 pachytene pairing in autotetraploids (Ch. IV). It happens through 

 abnormal genotypes reducing the precocity of prophase (Ch. X) and 

 through exceptional physiological conditions. The chiasmata which 

 hold the chromosomes together are interstitial at first. They 

 becomes terminal by movement. The evidence that terminal chias- 

 mata correspond in structure, in development and in function to the 

 interstitial chiasmata from which they are derived may be sum- 

 marised as follows : — 



1. The terminal connection between pairs of chromosomes can 

 always be seen to be double in favourable material, i.e., to be a 

 connection between pairs of chromatids not associated proximally. 

 It is therefore a change of partner amongst chromatids (Tulipa, 

 Newton and D., 1929 ; Tradescantia, D., 1929 c ; Primus, D., 1930 a ; 

 Primula, D., 1931 a ; (Enothera, D., 1931 d, Catcheside, 1931 : 

 Pyrus, D. and Moffett, 1930). 



2. The joint terminal connection between three and four 

 chromosomes (triple and quadruple chiasma) can be seen similarly 

 to be a change of partner amongst chromatids. The chromosomes 

 are not associated as such, but by the property of pairing (and 

 changing partner) possessed by their chromatids. By virtue of 

 this, one chromosome is never directly connected at the ends with 

 more than two others (D., 1929 c, 1931 a ; Meurman, 1929 a). 



3. Where the association between one pair of chromatids fails, 

 the " imperfect chiasma " resulting gives less resistance to anaphase 

 separation (D., 1929 c) and in a quadrivalent with a quadruple 

 chiasma gives rise to a special type of configuration where the 

 chromosomes are arranged in a line instead of in the cross (Fig. 42, 

 IV, c and d). 



4. The movement towards the end may be shown by comparing 

 the frequencies of chiasmata in difterent parts of the paired chromo- 

 somes at different stages {Phrynotettix, Primula, Matthiola, Rosa). 

 They are more frequent near the ends at the later stages. 



5. At metaphase, interstitial chiasmata may be occasionally 



